This invention concerns a method of manufacturing polyester fibers with good adhesion to rubber, particularly by using a finish composition which contains an ester of a thiodicarboxylic acid and an epoxy compound, followed by heat treatment of the polyester fibers.
Polyester fibers such as polyethylene terephthalate, have excellent physical and chemical properties, are mass produced industrially and are widely used in various fields. They are also a very suitable material for reinforcing rubber.
However, polyester fibers have a great defect in that they are inferior in adhesion to rubber when compared to polyamide fibers such as nylon 6, nylon 6,6, etc., which are also typical industrial fibers.
Thus when polyamide fibers are simply treated with a resorcinol-formaldehyde-rubber latex (RFL) adhesive, they have good adhesion to rubber, but in the case of polyester fibers, even when this RFL treatment is performed, good adhesion cannot be obtained.
For this reason, many attempts have been made to improve the adhesion of polyester fibers to rubber. Typical methods for doing this are (1) when treating the raw cord textile with the RFL treatment (the so-called "dipping treatment"), it is first pretreated with an adhesive such as an epoxy compound, an isocyanate compound, an ethylene urea compound, etc., and then treated with RFL or RFL mixed with an esterophilic ingredient such as a novolak resin, or 2,6-bis(2', 4'-dihydroxphenylmethyl)-4-chlorophenol, known by the trade name "Pexul"; and (2) an adhesive such as an epoxy compound, an isocyanate compound, etc., is added at the yarn stage, followed by dip-treating with RFL. Although the former method does give the desired adhesion to some degree, it has defects in that it requires a large quantity of adhesive, and the treatment method is troublesome, which tends to increase the cost. Although the latter method has the practical advantage that, like the polyamide fibers, the later dip treatment can be performed with RFL alone, it has the defect that the essential adhesive function is somewhat insufficient. Consequently, particularly in the latter method, a new problem is created that, in order to increase the adhesiveness, the method of treatihg the yarn itself must be performed under conditions which deviate greatly from the practical range: the concentration of the adhesive used in treating the yarn is markedly increased, the heat treatment conditions are made extremely severe, etc.
Many methods are already known for adding epoxy compounds when polyester fibers are spun. However, all of the previous methods have defects, e.g., (as mentioned above) their adhesive ability is not yet sufficient, or they require severe treatment conditions which greatly exceed what is practical; thus, they cannot be called satisfactory methods.